Istikhara is only a way of self-reflection and figuring out the pros and cons of something you intend to do. It is done by everybody on this earth; people who follow some religion, and people who follow none. It doesn't provide you any divine guidance (although people would coerce you into believing so). Its not some "abra cadabra" after which you become aware of what future has in store for you with respect to a certain decision.
You should simply weigh pros and cons of marrying someone and go with it if you feel its the right decision.
However, if you are having bad feelings about every proposal you come across, your mother may be right. It may have to do with some unknown fear you have about getting married. It may be a result of some past experience or some misguided notion you have about marriage or something else. Try to discuss it with somebody who you are close to and who you think can provide you with a sound advice. If you keep on living with this unknown fear, I am afraid all your istikharas would come to the same result.